Evoked potentials (EPs) are not a common topic on in-service or board examinations; Expect only a few questions on EPs on test day.
Medical students shouldn’t be expected to know much about evoked potentials after their neurology clerkship training. Neurology residents may have very little exposure to these diagnostic tests during their training. Because of this, it may be valuable to briefly cover the three most common evoked potential tests performed and seen on examinations: Brainstem auditory evoked potentials (BAEPs), Visual evoked potentials (VEPs), and Somatosensory Evoked Potentials (SSEPs).
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- Evoked potentials (EPs) are the electrical manifestation of the brain’s response to an external stimulus and can provide information regarding the functional integrity of sensory pathways. External sensory stimuli are somatosensory, auditory, or visual.
- Each evoked potential (visual, somatosensory, auditory) has typical measurable waves that represent particular regions along its particular pathway. Depending on the recording location, EP waves can be peripheral nerve, subcortical (brainstem), or cortical. The presence or absence of EP waves, as well as wave latency and amplitude, can help determine functional status.
- Each wave has a particular latency (msec) and amplitude (mV).
- Increased wave latencies indicate a conduction abnormality.
Brainstem Auditory Evoked Potentials (BAEPs)
- BAEPs are utilized in various neurosurgical operations including acoustic neuroma resection, as well as removal of other tumors or vascular abnormalities within the posterior fossa. They are not useful in monitoring deep thalamic or cerebral auditory pathways.
- BAEPs are produced by a brief auditory stimulus (click) to one ear.
- BAEPs evaluate the peripheral (cranial nerve VIII) and central auditory pathways (cochlear nucleus, superior olivary nucleus, lateral lemniscus, inferior colliculus, medial geniculate body, and thalamocortical pathways).
- Brainstem auditory evoked potentials (BAEPs) have seven measurable waves:
- Wave I: Cranial nerve VIII
- Wave II: Cochlear nucleus
- Wave III: Ipsilateral superior olivary nucleus
- Located in the lower pons
- Wave IV: Lateral lemniscus
- Wave V: Inferior colliculus
- Wave VI: Medial geniculate body
- Located in the thalamus
- Wave VII: Thalamocortical pathways
- Waves VI and VII are not regularly measured clinically.
- Waves I, III, and V are the most important clinically.
- These waves can be remembered as they are in numerical order of the natural transmission of signals through the auditory pathway.
- Abnormal time intervals between particular waves (interpeak latencies) correlate with various lesions within the brain-stem auditory tract.